In recent years, solar cells have been increasing importance as alternative energy for fossil fuels and as a measure against global warming. However, existing solar cells represented by silicon-based solar cells are expensive under the current circumstances, and the expensiveness is a factor that prohibits popularization.
Hence, research and development for various low-cost solar cells is promoted. Among such solar cells, dye-sensitized solar cells proposed by Graetzel et al. from Swiss Federal Institute of Technology in Lausanne are highly expected for reduction to practical use (see, e.g., Patent document 1 and Non-patent documents 1 and 2).
The structure of the solar cells is formed of a porous metal oxide semiconductor on a transparent conductive glass substrate; a dye adsorbed to a surface of the porous metal oxide semiconductor; an electrolyte containing a redox couple; and a counter electrode.
The dye-sensitized solar cells of Patent document 1 and Non-patent documents 1 and 2 are remarkably improved in photoelectric conversion efficiency by being increased in surface area using a porous material as the electrode formed of the metal oxide semiconductor such as titanium oxide and by having a ruthenium complex monomolecularly adsorbed as the dye.
Further, it is expected that the production costs can be reduced, because a printing technique can be applied as the method for producing the photoelectric conversion element and no expensive production facilities are needed hence. However, the solar cells contain iodine and a volatile solvent and have problems that the power generating efficiency may drop due to deterioration of the iodine redox system and that the electrolytic solution may volatilize or leak.
As compensation for these disadvantages, the following solid dye-sensitized solar cells are proposed:
1) Solid dye-sensitized solar cells using an inorganic semiconductor (see, e.g., Non-patent documents 3 and 4);
2) Solid dye-sensitized solar cells using a low-molecular-weight organic hole transport material (see, e.g., Patent document 2 and Non-patent documents 5 and 6); and
3) Solid dye-sensitized solar cells using a conductive polymer (see, e.g., Patent document 3 and Non-patent document 7).
The solar cell described in Non-patent document 3 uses copper iodide as a material to constitute a p-type semiconductor layer. This solar cell has a relatively good photoelectric conversion efficiency immediately after production, but it is known that the photoelectric conversion efficiency drops by half in a few hours due to deterioration attributed to, for example, increase of crystal grains of copper iodide.
Hence, the solar cell described in Non-patent document 4 additionally contains imidazolinium thiocyanato to suppress crystallization of copper iodide. However, the suppression is not sufficient.
The solid dye-sensitized solar cell that is described in Non-patent document 5 and is of a type using an organic hole transport material is reported by Hagen et al. and improved by Graetzel et al. (see Non-patent document 6).
The solid dye-sensitized solar cell described in Patent document 2 and using a triphenylamine compound includes a charge transport layer formed by vacuum vapor deposition of the triphenylamine compound.
Hence, the triphenylamine compound cannot reach internal voids in the porous semiconductor. Therefore, it has only been possible to obtain a low conversion efficiency.
In the example described in Non-patent document 6, a spiro-type hole transport material is dissolved in an organic solvent, and spin-coating is employed to obtain a composite body of nano-titania particles with the hole transport material.
In this solar cell, however, an optimum value of the thickness of a nano-titania particle film is specified to be about 2 μm, which is by far smaller than when an iodine electrolytic solution is used, i.e., from 10 μm through 20 μm. Hence, an amount of the dye adsorbed to the titanium oxide is low, to make it difficult to achieve sufficient light absorption and sufficient carrier generation. Hence, it is impossible to reach the characteristic obtained when an electrolytic solution is used.
As a solid solar cell of the type using a conductive polymer, a solid solar cell using polypyrrole has been reported by Yanagida et al. from Osaka University (see Non-patent document 7). This solid solar cell has also been able to obtain only a low conversion efficiency. A solid dye-sensitized solar cell described in Patent document 3 and using a polythiophene derivative includes a charge transfer layer formed by electrolytic polymerization on a dye-adsorbed porous titanium oxide electrode. However, there are problems that the dye may desorb from the titanium oxide and that the dye may decompose. Furthermore, the polythiophene derivative is considerably problematic in durability.
Owing to recent technological developments, driving power for electronic circuits has been significantly reduced, and it has become possible to drive various electronic parts such as sensors by converting weak light such as room light to electricity.
Further, it has been reported that existing electrolytic solution-type dye-sensitized solar cells using, for example, iodine, have a photoelectric conversion characteristic greater than or equal to amorphous silicon solar cells under weak room light (see Non-patent document 8).
However, the electrolytic solution-type dye-sensitized solar cells contain iodine and a volatile solvent described above and have problems that the power generating efficiency may drop due to deterioration of the iodine redox system and that the electrolytic solution may volatilize or leak.
It has also been reported that when weak light such as room light is converted to electricity, loss current due to an internal resistance in the photoelectric conversion element is conspicuous (see Non-patent document 9).
When the internal resistance is raised, a short-circuiting current density worsens to degrade the photoelectric conversion characteristic. When the internal resistance is lowered, an open circuit voltage worsens to degrade the photoelectric conversion characteristic. That is, it is extremely difficult to satisfy both of raising the internal resistance; and a good photoelectric conversion characteristic.
An open circuit voltage obtained with the photoelectric conversion element under weak light such as the room light is lower than under pseudo sunlight. Hence, in order to obtain an output voltage needed for driving an electronic circuit, there is a need for obtaining a high open circuit voltage.
Hitherto, there have been reported basic substances that can achieve a high open circuit voltage (see Non-patent document 10). However, there is no basic material that can achieve a photoelectric conversion characteristic better than hitherto used 4-tertial butylpyridine in a dye-sensitized solar cell of the type using an electrolytic solution such as iodine.
As described above, under the current circumstances, none of the solid photoelectric conversion elements studied so far have been able to obtain a satisfactory characteristic.